U.S. patent application number 10/846306 was filed with the patent office on 2005-11-17 for system and method for calibrating electronic circuitry.
Invention is credited to Summers, James B..
Application Number | 20050253574 10/846306 |
Document ID | / |
Family ID | 35308811 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050253574 |
Kind Code |
A1 |
Summers, James B. |
November 17, 2005 |
SYSTEM AND METHOD FOR CALIBRATING ELECTRONIC CIRCUITRY
Abstract
In one embodiment, an algorithm is employed to calibrate
electronic circuitry that is adjustable according to operating
parameters. The calibration causes an output component to be
substantially nulled. The algorithm comprises adjusting at least
one of the operating parameters over a plurality of values,
applying an input signal to the electronic circuitry concurrently
with the adjusting, measuring magnitudes of the output component
from the electronic circuitry produced in response to the input
signal and the adjusting, wherein the plurality of values are
selected such that the magnitudes are not substantially nulled, and
providing the magnitudes and the plurality of values to a curve
fitting algorithm to calculate a plurality of operating parameters
that cause the output component to be substantially nulled.
Inventors: |
Summers, James B.; (Spokane
Valley, WA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.
Legal Department, DL429
Intellectual Property Administration
P.O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
35308811 |
Appl. No.: |
10/846306 |
Filed: |
May 14, 2004 |
Current U.S.
Class: |
455/302 |
Current CPC
Class: |
H03C 3/406 20130101 |
Class at
Publication: |
324/158.1 |
International
Class: |
G01R 027/00 |
Claims
What is claimed is:
1. A method for calibrating electronic circuitry, that is
adjustable according to operating parameters, to cause an output
component to be substantially nulled, comprising: adjusting at
least one of said operating parameters over a plurality of values;
applying an input signal to said electronic circuitry concurrently
with said adjusting; measuring magnitudes of said output component
from said electronic circuitry produced in response to said input
signal and said adjusting, wherein said plurality of values are
selected such that at least some of said magnitudes are not
substantially nulled; and providing said magnitudes and said
plurality of values to a curve fitting algorithm to calculate a
plurality of operating parameters that cause said output component
to be substantially nulled, wherein said curve fitting algorithm is
defined by at least one equation modeling a response of said
electronic circuitry.
2. The method of claim 1 further comprising: varying a frequency of
said input signal; and repeating said adjusting, applying,
measuring, and providing in response to said varying.
3. The method of claim 2 further comprising: storing calculated
operating parameters for each frequency selected for said input
signal.
4. The method of claim 1 wherein one of said operating parameters
defines a gain value of a variable gain amplifier.
5. The method of claim 1 wherein one of said operating parameters
defines a phase relationship between signals processed by said
electronic circuitry.
6. The method of claim 1 wherein one of said operating parameters
defines a direct-current (DC) offset.
7. The method of claim 1 wherein said electronic circuitry is an
image reject mixer and said output component is an image component
of said image reject mixer.
8. The method of claim 1 wherein said at least one equation
modeling a response does not define sensitivities of said
electronic circuitry in response to changes of said operating
parameters.
9. The method of claim 8 wherein said curve fitting algorithm
calculates said sensitivities of said electronic circuitry and
calculates said plurality of operating parameters using said
calculated sensitivities and errors determined from said
magnitudes.
10. A system for calibrating electronic circuitry, that is
adjustable according to operating parameters, to cause an output
component to be substantially nulled, comprising: means for varying
an operating parameter; means for causing application of an input
signal to said electronic circuitry concurrently with operation of
said means for varying; means for obtaining measurements of said
output component from said electronic circuitry produced in
response to said input signal, wherein means for varying does not
cause said output signal to be substantially nulled for some of
said measurements; and means for calculating a plurality of
operating parameters that cause said output component to be
substantially nulled using a curve fitting algorithm that processes
said measurements and said varied operating parameter, wherein said
curve fitting algorithm is characterized by at least one equation
modeling a response of said electronic circuitry.
11. The system of claim 10 further comprising: means for varying a
frequency of said input signal.
12. The system of claim 11 further comprising: means for storing
calculated operating parameters for each frequency applied by said
means for varying.
13. The system of claim 10 wherein said operating parameter defines
a gain of a variable gain amplifier.
14. The system of claim 10 wherein said operating parameter defines
a phase relationship between signals processed by said electronic
circuitry.
15. The system of claim 10 wherein said operating parameter defines
a direct-current (DC) offset applied to a signal processed by said
electronic circuitry.
16. An image reject mixer, comprising: a first mixer for mixing an
input signal and an I-channel; a second mixer for mixing said input
signal and a Q-channel; a summer for mixing outputs from said first
and second mixers; a variable gain amplifier for amplifying one of
said I-channel and said Q-channel; a delay element for modifying a
phase relationship between said I-channel and said Q-channel; a
controller for calibrating operation of said image reject mixer by:
(i) varying operation of said delay element; (ii) obtaining signal
measurements from said summer in coordination with said varying,
wherein at least some measurements are not substantially nulled;
(iii) applying said measurements to a curve fitting algorithm to
calculate operating parameters for said variable gain amplifier and
said delay element to substantially null an image product of said
image reject mixer, wherein said curve fitting algorithm is
characterized by a plurality of equations modeling a response of
said image reject mixer.
17. The image reject mixer of claim 16 wherein said controller is
operable to store calculated operating parameters for a plurality
of frequencies of said input signal.
18. The image reject mixer of claim 16 wherein when a frequency is
subsequently selected for operation of said image reject mixer that
is not associated with stored operating parameters, said controller
interpolates operating parameters using stored operating parameters
adjacent to said selected frequency.
19. The image reject mixer of claim 16 further comprising: a
digital frequency synthesizer for generating said I-channel and
said Q-channel.
20. The image reject mixer of claim 19 wherein said variable gain
amplifier is implemented in a digital domain using said digital
frequency synthesizer.
Description
TECHNICAL FIELD
[0001] The present invention is generally related to the
calibration of electronic circuitry.
BACKGROUND OF THE INVENTION
[0002] A mixer is a device that is designed to receive two
frequency signals and combines the signals to generate mixing
products at frequencies that are the sum and difference of the
received frequencies. In many cases, only one of the mixing
products is desired. The other product is referred to as the
"image." Image reject mixers are devices that ideally produce only
the sum or difference product, but not both. The implementation of
an image reject mixer involves two discrete mixers. The phase of
the signals applied to the two mixers is controlled such that, when
the two mixer outputs are combined, the desired products of the
mixers add constructively and the image products add destructively.
In the ideal case, the image is completely cancelled leaving only
the desired product.
[0003] For image reject mixers, two factors contribute to the
degree of image rejection. The first factor is the gain balance
between the two discrete mixers within the device. Specifically, if
one of the two discrete mixers generates products of greater
magnitude than the other mixer, the image products of the two
mixers will not completely cancel each other. The second factor is
the phase relationship associated with the mixing. Specifically, if
there is a phase error in the quadrature phase relationship between
the signals driving the mixers, the image output products of the
two mixers will not be exactly 180 degrees out of phase and hence
will not completely cancel each other.
[0004] To achieve a relatively high degree of image rejection, the
gain of the discrete mixers and the quadrature angle relationships
in an image reject mixer can be calibrated. Typically, an iterative
approach is applied in which the gain and the phase relationship
are repetitively adjusted and the magnitude of the image is
measured. Eventually, operating values defining the gain and phase
relationship can be located that substantially null the image
product. Additionally, a number of mathematical techniques can be
employed to cause the operating values to converge more quickly.
These techniques are generally related to "Newton's Method" of root
finding in which the derivative of the image product magnitude
versus the gain and phase are used to find the null.
BRIEF SUMMARY OF THE INVENTION
[0005] Representative embodiments are directed to systems and
methods for calibrating electronic circuitry. In one representative
embodiment, each operating parameter of the electronic circuitry is
adjusted over a plurality of values. The signal component to be
nulled is measured during the adjustment over the plurality of
values. The selection occurs such that the signal component is not
nulled during the measurement process for at least some of the
plurality of values. The measurement values and the plurality of
values used for the adjustment are provided to a curve fitting
algorithm. The curve fitting algorithm is characterized by the
response of the electronic equipment to the operating parameters.
The curve fitting algorithm calculates optimal operating parameters
for the electronic circuitry. By calibrating the electronic
circuitry according to the calculated parameters, the signal
component will be substantially nulled. Although one embodiment
employs a curve fitting algorithm to calibrate an image reject
mixer, any suitable electronic circuitry that operates according to
a plurality of parameters can be calibrated by embodiments of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 depicts an image reject mixer with calibration
functionality according to one representative embodiment.
[0007] FIG. 2 depicts a flowchart for calibration of an image
reject mixer according to one representative embodiment.
[0008] FIG. 3 depicts a flowchart for calibration of electronic
circuitry according to one representative embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0009] To facilitate the discussion of one representative
embodiment, reference is made to image reject mixer 100 of FIG. 1.
The mixing functionality of image reject mixer 100 is typical of
known mixer designs. However, image reject mixer 100 differs from
known designs by employing controller 109 that calibrates mixer 100
according to one representative embodiment. Controller 109 may be
implemented to perform calibration of mixer 100 using calibration
algorithm 110 (e.g., suitable software instructions or integrated
circuitry functionality).
[0010] Image reject mixer 100 receives local oscillator input
signal 101 to be mixed with a digitally synthesized signal. Digital
frequency synthesizer 102 digitally synthesizes this signal and
converts the digital signal into analog form. Digital frequency
synthesizer 102 outputs a first version of the signal via line
103-1 and a second version of the signal via line 103-2 (i.e., I
and Q channels). The two versions of the signal are out of phase by
90.degree.. The first version of the signal can be either amplified
or attenuated by variable gain amplifier (VGA) 105. As shown in
FIG. 1, although VGA 105 is implemented within the digital domain,
an analog amplifier could be employed if desired. The gain of VGA
105 is represented by .DELTA..sub.LO. After VGA 105, the first
version of the signal is mixed with the LO signal 101 by mixer
104-1. The second version of the signal is mixed with a 90.degree.
phase shifted version of the LO signal 101 by mixer 104-2. The
mixed signals from mixers 104-1 and 104-2 are combined by adder 107
to produce image rejected mixer output 106. In the ideal case, the
image products are equal in amplitude and 180.degree. out of phase
and, hence, add destructively. The desired products are in phase
and hence add constructively.
[0011] In practice, several adjustments are typically needed.
Specifically, the two versions of the LO signal may not be
precisely 90.degree. out of phase. QuadDAC 108 provides an analog
signal which controls the phase shifter 108-1 in the LO signal path
to mixer 104-1. This allows the phase relationship between the LO
signals in the two mixers to be adjusted. Also, the gain associated
with mixers 104-1 and 104-2 may be unequal. VGA 105 may be used to
equalize the gain between mixers 104-1 and 104-2.
[0012] As previously discussed, known techniques iteratively vary
the gain and phase relationship to converge to a precise
calibration. The iterative approach is problematic. Specifically,
because the iterative techniques cause the measurement of the image
product to converge to a null, multiple measurements are made very
close to the null. Accordingly, a requirement of a wide dynamic
range is imposed by known techniques upon the measurement
mechanism. Furthermore, noise may cause errors in the measurements
and lengthen the amount of time to converge to the null. Moreover,
if the dynamic range is not sufficiently wide, noise may prevent
the algorithm from converging to the null.
[0013] In contrast to known techniques, controller 109 does not
attempt to converge to a null in an iterative manner. Instead,
controller 109 operates mixer 100 according to a plurality of
values for the setting of QuadDAC 108 and the gain of VGA 105.
Controller 109 measures the resulting signal characteristics (image
product and desired product amplitudes) using receiver 112. Based
upon the measurements, controller 109 employs a curve fitting
algorithm to calculate appropriate phase and gain settings. Only a
relatively few number of measurements are taken. Also, the
measurements need not be made very close to the null. Accordingly,
a wide dynamic range for the measurement mechanism is not
required.
[0014] To apply a curve fitting algorithm to image reject mixer 100
as shown in FIG. 1, the relationship between the image product and
the desired mixer product is defined as follows:
(Image Product Magnitude).sup.2=K.sub.RF(1+G.sup.2-2G cos(.PHI.)),
(Equation 1)
(Desired Product Magnitude).sup.2=K.sub.RF(1+G.sup.2+2G
cos(.PHI.)), (Equation 2)
[0015] where K.sub.RF is a gain constant, G is the gain imbalance
(where zero imbalance is represented by G=1), and .PHI. is the
phase error between the I and Q channels.
[0016] The values of G and .PHI. are determined in the calibration
process. To compensate for gain imbalance, it is relatively
straight forward to determine the proper hardware setting.
Specifically, the gain of VGA 105 is set to the reciprocal of G.
Compensation for the phase imbalance (.PHI.) is more complicated.
The phase imbalance can be modeled as:
.PHI.=.PHI..sub.ERROR+K.sub.QUAD*QuadDAC Setting,
[0017] where .PHI..sub.ERROR and K.sub.QUAD are unknown and LO
frequency dependent. K.sub.QUAD is the sensitivity of the LO phase
adjustment means 108-1. For perfect cancellation, .PHI. must be
zero so:
QuadDAC Setting=-.PHI..sub.ERROR/K.sub.QUAD.
[0018] K.sub.QUAD can vary for wide ranges of the QuadDAC setting.
Accordingly, the measurements are made by limiting the range of
variation in QuadDAC values to values "close" to the correct
calibration value to avoid the complication presented by this
variation. Measurements are made by setting the gain of VGA 105 and
the QuadDAC setting to devitate around the correct calibration
parameter. By taking the measurements in this manner, the
measurements can be made away from the image product null, lowering
the required dynamic range of the measurement mechanism. As a
result, noise will have a lesser effect on the calibration
process.
[0019] It is possible to dispense with K.sub.RF because it is
common to both the desired product and the image product.
Accordingly, the calibration process estimates three unknowns (G,
.PHI..sub.ERROR, and K.sub.QUAD).
[0020] According to one representative embodiment, five
measurements are made with different values of QuadDAC. A minimum
squared error algorithm is used to determine the values of the
unknowns that produce the minimum squared error as characterized by
equations (1) and (2). The appropriate value for QuadDAC is
determined from K.sub.QUAD and .PHI..sub.ERROR. The gain imbalance
(G) is not uniquely determined by only varying the QuadDAC setting.
Either G or 1/G will be the correct value. Additional measurements
using these two values can be made to resolve the ambiguity.
[0021] Due to measurement limitations associated with some receiver
mechanisms, the gain value generated by the proceeding operations
may not be as completely accurate as possible. A gain refinement
algorithm may be employed after the proceeding operations to
achieve a greater degree of accuracy if appropriate for a
particular application. Specifically, the following "residual" gain
error can be computed from the suppression generated by the
proceeding operations: g.sub.RESIDUAL=1.+-.sqrt(image product
magnitude/desired product magnitude). One of the values may be
selected to further adjust the gain of VGA 105. If the selected
value does not improve the image rejection, the other value is
known to accurately represent the residual gain error.
[0022] In one embodiment, the calibration process is performed for
a number of LO frequencies. The calibration parameters determined
for the LO frequencies are stored in calibration parameters 111.
When a user subsequently selects a LO frequency suitable for a
particular application, controller 109 retrieves the corresponding
calibration parameters and sets the hardware of mixer 100
accordingly. If calibration parameters are not found for a
particular LO frequency, interpolated parameter values may be
used.
[0023] FIG. 2 depicts a flowchart for calibration of an image
reject mixer across a plurality of LO frequencies according to one
representative embodiment. The flowchart of FIG. 2 could be
implemented as software instructions for controller 109 as an
example.
[0024] In step 201, a digital synthesizer is set for continuous
wave (CW) operation and the level of the synthesizer is set. In
step 202, the IF frequency (f.sub.IF) is set equal to 10 MHz. In
step 203, the receiver is set for loopback operation and
appropriate gain ranging so that it is measuring the output of the
mixer.
[0025] As previously discussed, it is advantageous to cause the
setting of the QuadDAC register to be "close" to the calibration
value during the measurement process. Accordingly, each iteration
of the flowchart uses the previously calculated calibration value
as the "center" value for the next set of measurements. However,
during the first iteration, a previous value of QuadDAC is not
available. Multiple iterations are performed for the first f.sub.LO
value to address the unavailability of a prior value of
QuadDAC.
[0026] To provide the multiple iterations, a logical comparison is
made (step 204) to determine whether the iteration of the process
flow is the first pass at the first value of f.sub.LO. If true, the
process flow proceeds to step 205 where the value of QuadDAC is set
to 2048 (its midrange value). This initial setting is dependent
upon the particular hardware used to implement the digital to
analog converstion signal which drives the phase adjustment. If the
logical comparison of step 204 is false, the process flow proceeds
to step 206 where the value of QuadDAC is set to the value of
QuadDACCal associated with the previous iteration of the process
flow. QuadDACCal is a variable that holds the value calculated by
the curve fitting process for the correct hardware setting of
QuadDAC.
[0027] In step 207, the LO frequency synthesizer 101 is set to
achieve a LO signal frequency of f.sub.LO. The variable f.sub.LO,
the local oscillator frequency, is set to an initial value. The
variable f.sub.LO is stepped over a range of frequencies to cause
the calibration process to be repeated to address the
frequency-dependent nature of the image reject mixer. In step 208,
multiple measurements of the image rejection are made by setting
QuadDAC to the following values: the center value (see steps 205
and 206), the center value.+-.100, and the center value.+-.200.
These settings of the phase adjustment parameters depend upon the
implementation of the quadrature adjustment means. By varying the
settings in this manner, a number of measurements will be made that
are not substantially nulled. Accordingly, the calibration will be
relatively robust against noise and does not require wide dynamic
range.
[0028] In step 209, a curve fitting algorithm is performed to
calculate .PHI..sub.ERROR, K.sub.QUAD, and G. For example, a
minimum squared error algorithm may be applied to calculate the
respective values. In step 210, the variable QuadDACCal is
calculated from: the center value-.PHI..sub.ERROR/K.sub.QUAD. The
variable .DELTA..sub.LOcal is set to equal G.
[0029] In step 211, the image rejection values are measured in
association with setting the register QuadDAC to equal QuadDACCal
and the gain value .DELTA..sub.LO to equal .DELTA..sub.LOcal and
1/.DELTA..sub.LOcal. In step 212, a logical comparison is made to
determine whether .DELTA..sub.LOcal produces a greater amount of
image rejection than 1/.DELTA..sub.LOcal. If not, the process flow
proceeds to step 213 where the variable .DELTA..sub.LOcal is set to
equal the reciprocal. If so, the process proceeds to step 214.
[0030] In step 214, the image rejection ratio (r.sub.1) is measured
by setting QuadDAC to QuadDACCal and .DELTA..sub.LO to
.DELTA..sub.LOcal. In step 215, a gain residual variable
(g.sub.RESIDUAL) is set to equal 1+sqrt(r.sub.R1). In step 216,
.DELTA..sub.LO is set to equal .DELTA..sub.LOcal*g.sub.RESIDUAL. In
step 217, the resulting image rejection ratio (r.sub.2) is
measured.
[0031] In step 218, a logical comparison is made to determine which
image rejection ratio (r.sub.1 or r.sub.2) is greater. If r.sub.R1
is greater, the process flow proceeds to step 219. In step 219, the
variable g.sub.RESIDUAL is set to equal 1-sqrt (r.sub.1). In step
220, the variable .DELTA..sub.LOcal is set to equal
.DELTA..sub.LOcal*g.sub.RESIDU- AL.
[0032] In step 221, a logical comparison is made to determine
whether the last frequency of the desired range of LO signal
frequencies has been examined. If not, the process flow proceeds to
step 222. If the last frequency has been examined, the parameters
for the last iteration are stored (step 225) and the process ends
(step 226).
[0033] In step 222, a logical comparison is made to determine
whether the process flow is associated with the first past for the
first value of f.sub.LO. If so, the process flow returns to step
203 to perform another pass for the first value of f.sub.LO to
further refine the value of QuadDACcal. If not, the process flow
proceeds to step 223. In step 223, the values of QuadDACcal and
.DELTA..sub.LOcal are stored as the appropriate calibration
parameters for the current value of f.sub.LO. In step 224, the
value of f.sub.LO is incremented by 25 MHz. From step 224, the
process flow returns to step 203.
[0034] In an alternative embodiment, LO leakage may be addressed in
a manner similar to the calibration described in FIG. 2. In
general, some phase and amplitude components will leak through each
mixer 104 in a frequency dependent manner. The LO leakage is
especially pronounced when upconverting a relatively low frequency
to a relatively high frequency. Specifically, the LO leakage will
be close to the desired signal and hence difficult to filter.
Adding DC offsets to the I and Q signals (following the variable
gain on the Q channel) allows the LO leakage to be cancelled by
providing an intentional leakage vector with amplitude equal to
that of the mixer leakage component and with opposite phase. The
appropriate values for the DC offsets can be calculated using
multiple measurements and a curve fitting algorithm according to
one representative embodiment.
[0035] Although some embodiments have been described in terms of
calibrating an image reject mixer, the present invention is not so
limited. Other representative embodiments can be used to calibrate
any suitable type of electronic circuitry where the mathematical
relationship between the calibration adjustment parameters and
circuit performance is known, but where coefficients in the
relationship (such as gains and offsets) are not. For example, FIG.
3 depicts a flowchart for calibrating electronic circuitry that is
adjustable using operating parameters according to one
representative embodiment. The calibration is performed to null a
signal or signal component (the "output product") generated by the
electronic circuitry. The process flow of FIG. 3 may be implemented
using a processor and suitable software instructions and/or using
integrated circuit functionality.
[0036] In step 301 of FIG. 3, at least one of the operating
parameters is adjusted over a plurality of values. The operating
parameters are selected such that the output product is not
substantially nulled during the measurement process for at least
some of the operating parameters. In the present context, the term
"not substantially nulled" means that the resulting signal
characteristic or characteristics are sufficiently above the noise
threshold to enable accurate measurement using the available
measurement functionality. In step 302, an input signal is applied
to the electronic circuitry in a concurrent manner with the
adjustment of the operating parameters. In step 303, magnitudes of
the output product from the electronic circuitry are measured that
occur in response to the input signal and the plurality of
operating parameters. In step 304, the magnitudes of the output
product and the plurality of values are provided to a curve fitting
algorithm. The curve fitting algorithm employs one or several
equations that characterize the electronic circuitry being
calibrated. The curve fitting algorithm calculates a plurality of
operating parameters that cause the output product to be
substantially nulled. The calculated operating parameters are used
to calibrate various elements of the electronic circuitry to cause
the output product to be substantially nulled during operation of
the electronic circuitry.
[0037] By employing a curve fitting algorithm, some representative
embodiments enable calibration of electronic circuitry to occur in
a relatively efficient manner. Numerous iterations are not required
to converge to "optimal" settings. Furthermore, some representative
embodiments do not require a relatively large dynamic range on the
measurement mechanism used to calibrate the electronic circuitry.
Additionally, some representative embodiments perform the
calibration process in a manner that is relatively robust to noise
in the measurement data.
* * * * *